Red Top
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Red Top clear sky chart from 2005-11-06 to 2025-10-19.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	12% 9% 3 8% 5% 4 5% 4 5% 7% 37%	127829
	Night	12% 9% 4 9% 5% 5% 5% 4 5% 6% 36%	122066
Transparency	Day	45% 5% 13% 25% 11% 1	131374
	Night	47% 5% 13% 23% 12% 1	126598
Seeing	Day	16% 8% 28% 29% 16% 4	17433
	Night	16% 8% 31% 32% 12% 1	27253

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	7% 13% 5% 14% 10% 5% 5% 3 4 6% 27%	3422
	11th to 20th	Day	5% 10% 6% 14% 8% 7% 8% 8% 10% 6% 17%	3620
	21st to end	Day	5% 15% 9% 16% 6% 5% 5% 3 5% 6% 26%	4417
February	1st to 10th	Day	8% 13% 4 10% 6% 7% 5% 4 6% 7% 30%	1800
	11th to 20th	Day	7% 11% 4 10% 6% 4 5% 3 6% 7% 36%	1754
	21st to end	Day	8% 12% 4 10% 5% 5% 6% 4 5% 8% 33%	1522
March	1st to 10th	Day	11% 9% 3 7% 4 4 4 4 5% 8% 41%	4815
	11th to 20th	Day	9% 8% 3 9% 6% 5% 4 4 6% 8% 37%	4980
	21st to end	Day	20% 12% 4 9% 4 4 4 3 5% 6% 29%	5525
April	1st to 10th	Day	15% 12% 3 8% 5% 5% 4 3 6% 5% 33%	2400
	11th to 20th	Day	17% 12% 5% 8% 5% 4 4 3 5% 6% 31%	2422
	21st to end	Day	21% 10% 3 7% 5% 4 5% 3 4 5% 32%	2600
May	1st to 10th	Day	14% 7% 2 6% 3 3 3 3 4 5% 50%	6197
	11th to 20th	Day	17% 9% 2 6% 4 3 4 3 4 6% 42%	6319
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	6935
June	1st to 10th	Day	10% 6% 2 5% 4 4 4 4 7% 9% 44%	2800
	11th to 20th	Day	14% 7% 3 7% 4 4 5% 4 6% 7% 39%	2995
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 9% 39%	3000
July	1st to 10th	Day	8% 4 2 5% 4 5% 5% 4 7% 11% 45%	6800
	11th to 20th	Day	10% 4 2 5% 3 4 4 4 6% 8% 50%	6448
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 10% 48%	6924
August	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 6% 8% 43%	4267
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 5% 8% 41%	4171
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 5% 7% 43%	4515
September	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 10% 45%	2400
	11th to 20th	Day	12% 5% 2 7% 4 4 5% 4 5% 7% 45%	2366
	21st to end	Day	13% 8% 2 5% 4 4 4 4 5% 7% 44%	2106
October	1st to 10th	Day	16% 9% 2 7% 4 3 4 4 6% 7% 38%	2000
	11th to 20th	Day	15% 14% 3 8% 4 5% 4 3 4 6% 34%	1980
	21st to end	Day	19% 12% 4 9% 4 3 4 4 4 5% 33%	2043
November	1st to 10th	Day	13% 13% 5% 10% 5% 5% 5% 4 4 5% 32%	1600
	11th to 20th	Day	12% 15% 7% 13% 8% 4 4 4 4 5% 24%	1600
	21st to end	Day	13% 12% 5% 9% 6% 6% 5% 4 5% 7% 28%	1600
December	1st to 10th	Day	8% 13% 5% 13% 16% 7% 5% 4 5% 5% 19%	2610
	11th to 20th	Day	8% 16% 8% 13% 6% 5% 4 4 4 5% 27%	3008
	21st to end	Day	18% 20% 7% 21% 6% 4 3 2 3 4 12%	3868
January	1st to 10th	Night	8% 8% 4 16% 11% 6% 5% 3 5% 5% 30%	6487
	11th to 20th	Night	6% 10% 5% 12% 7% 8% 9% 8% 8% 7% 21%	6360
	21st to end	Night	6% 15% 8% 14% 5% 5% 4 4 5% 7% 27%	6559
February	1st to 10th	Night	7% 10% 5% 10% 7% 6% 6% 3 5% 7% 35%	2600
	11th to 20th	Night	6% 10% 4 10% 6% 6% 5% 4 6% 7% 37%	2533
	21st to end	Night	10% 11% 4 9% 6% 5% 5% 4 6% 7% 33%	2106
March	1st to 10th	Night	11% 7% 2 7% 4 4 4 3 4 6% 49%	5080
	11th to 20th	Night	10% 8% 3 10% 5% 5% 6% 4 6% 7% 34%	4980
	21st to end	Night	21% 12% 4 11% 4 4 3 3 4 4 29%	5389
April	1st to 10th	Night	17% 11% 3 7% 5% 3 4 3 5% 6% 36%	2000
	11th to 20th	Night	19% 12% 4 7% 4 4 3 3 4 5% 37%	1924
	21st to end	Night	19% 9% 3 8% 4 4 4 4 4 7% 33%	1800
May	1st to 10th	Night	11% 8% 2 5% 3 3 3 3 4 6% 51%	3715
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 5% 6% 48%	3596
	21st to end	Night	15% 9% 3 7% 4 4 4 4 5% 7% 38%	3942
June	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 7% 8% 44%	1600
	11th to 20th	Night	10% 7% 2 6% 5% 5% 4 4 6% 7% 45%	1400
	21st to end	Night	9% 7% 2 6% 5% 4 4 4 6% 6% 48%	1400
July	1st to 10th	Night	8% 4 2 4 4 4 4 4 7% 10% 50%	3178
	11th to 20th	Night	9% 3 1 5% 3 4 3 3 5% 6% 57%	3527
	21st to end	Night	10% 4 3 6% 5% 4 4 3 6% 8% 47%	3947
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 6% 8% 50%	2890
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 6% 7% 49%	2888
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	3389
September	1st to 10th	Night	8% 5% 2 5% 4 3 4 3 5% 8% 53%	2000
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 6% 8% 51%	2030
	21st to end	Night	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2273
October	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 5% 6% 46%	2400
	11th to 20th	Night	15% 11% 3 7% 4 4 4 4 5% 6% 38%	2377
	21st to end	Night	17% 11% 4 9% 4 4 4 4 5% 6% 32%	2555
November	1st to 10th	Night	14% 11% 3 9% 5% 4 5% 4 5% 6% 36%	2698
	11th to 20th	Night	15% 11% 5% 10% 5% 6% 5% 4 5% 6% 29%	2800
	21st to end	Night	12% 9% 4 8% 7% 7% 5% 4 6% 6% 32%	2800
December	1st to 10th	Night	6% 9% 5% 18% 10% 8% 6% 5% 6% 6% 20%	4580
	11th to 20th	Night	9% 16% 6% 15% 7% 5% 6% 3 5% 6% 23%	5199
	21st to end	Night	20% 19% 6% 15% 5% 4 3 2 3 4 17%	7064

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	58% 4 9% 16% 14%	3423
	11th to 20th	Day	59% 6% 8% 18% 9%	3620
	21st to end	Day	57% 6% 12% 17% 7%	4417
February	1st to 10th	Day	52% 4 11% 22% 10% 1	1800
	11th to 20th	Day	46% 6% 12% 25% 10% 1	1763
	21st to end	Day	49% 5% 12% 21% 12% 1	1522
March	1st to 10th	Day	41% 5% 15% 24% 11% 4	4815
	11th to 20th	Day	44% 4 14% 27% 11% 1	4980
	21st to end	Day	55% 6% 9% 21% 9% 1	5527
April	1st to 10th	Day	50% 5% 11% 19% 13% 2	2400
	11th to 20th	Day	53% 5% 10% 19% 12% 1	2422
	21st to end	Day	53% 5% 11% 20% 11%	2600
May	1st to 10th	Day	36% 4 13% 26% 18% 2	6237
	11th to 20th	Day	43% 5% 12% 22% 11% 6%	6323
	21st to end	Day	45% 7% 9% 26% 13%	6949
June	1st to 10th	Day	34% 8% 16% 31% 10% 1	2800
	11th to 20th	Day	43% 7% 14% 22% 13% 1	2995
	21st to end	Day	37% 9% 16% 27% 11% 1	3000
July	1st to 10th	Day	34% 4 21% 31% 10%	6726
	11th to 20th	Day	32% 5% 16% 35% 12% 1	6371
	21st to end	Day	31% 4 19% 32% 13%	6728
August	1st to 10th	Day	38% 6% 13% 28% 14% 1	4252
	11th to 20th	Day	41% 5% 14% 32% 9%	4210
	21st to end	Day	41% 4 16% 26% 11% 1	4514
September	1st to 10th	Day	34% 7% 15% 35% 9%	2400
	11th to 20th	Day	38% 7% 19% 28% 8%	2366
	21st to end	Day	39% 7% 13% 31% 9% 1	2106
October	1st to 10th	Day	35% 4 11% 34% 15% 1	3260
	11th to 20th	Day	43% 4 10% 24% 18% 1	3220
	21st to end	Day	51% 3 16% 23% 7%	3345
November	1st to 10th	Day	55% 5% 11% 19% 10% 1	1600
	11th to 20th	Day	62% 6% 14% 13% 5% 1	1600
	21st to end	Day	55% 8% 17% 15% 4 2	1600
December	1st to 10th	Day	68% 5% 10% 13% 3 1	2610
	11th to 20th	Day	60% 4 9% 18% 10%	3005
	21st to end	Day	77% 3 7% 8% 5%	3868
January	1st to 10th	Night	55% 4 10% 17% 13% 1	6487
	11th to 20th	Night	57% 5% 11% 14% 11% 1	6360
	21st to end	Night	55% 5% 14% 20% 6% 1	6559
February	1st to 10th	Night	48% 5% 12% 22% 12% 1	2600
	11th to 20th	Night	45% 5% 13% 24% 11% 2	2534
	21st to end	Night	48% 5% 13% 20% 12% 1	2106
March	1st to 10th	Night	38% 4 16% 21% 18% 4	5080
	11th to 20th	Night	47% 3 14% 23% 13%	4980
	21st to end	Night	60% 3 9% 15% 12% 1	5389
April	1st to 10th	Night	48% 4 10% 20% 16% 2	2000
	11th to 20th	Night	51% 3 9% 20% 15% 1	1924
	21st to end	Night	50% 6% 9% 20% 13% 2	1800
May	1st to 10th	Night	35% 4 13% 29% 18% 1	3740
	11th to 20th	Night	37% 4 12% 24% 16% 7%	3597
	21st to end	Night	45% 6% 10% 26% 13% 1	3950
June	1st to 10th	Night	36% 7% 17% 27% 11% 2	1600
	11th to 20th	Night	37% 7% 14% 23% 17% 2	1400
	21st to end	Night	35% 8% 15% 29% 12% 2	1400
July	1st to 10th	Night	30% 5% 20% 31% 13%	3145
	11th to 20th	Night	28% 3 15% 38% 13% 2	3485
	21st to end	Night	36% 4 16% 30% 13% 1	3832
August	1st to 10th	Night	32% 4 16% 28% 17% 3	2881
	11th to 20th	Night	35% 4 13% 36% 11% 1	2915
	21st to end	Night	39% 5% 17% 28% 10% 1	3389
September	1st to 10th	Night	29% 7% 16% 35% 12% 1	2000
	11th to 20th	Night	32% 7% 18% 32% 12%	2030
	21st to end	Night	35% 6% 12% 34% 12% 1	2273
October	1st to 10th	Night	27% 3 8% 36% 22% 4	3920
	11th to 20th	Night	40% 4 11% 26% 19% 1	3876
	21st to end	Night	49% 3 14% 22% 11%	4203
November	1st to 10th	Night	50% 5% 13% 22% 8% 2	2698
	11th to 20th	Night	56% 7% 13% 15% 9%	2800
	21st to end	Night	51% 8% 16% 18% 5% 1	2800
December	1st to 10th	Night	63% 6% 15% 13% 3 1	4580
	11th to 20th	Night	63% 4 9% 15% 8% 1	5201
	21st to end	Night	72% 3 6% 13% 6%	7064

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 11% 33% 28% 13% 2	300
	11th to 20th	Day	13% 9% 37% 35% 6%	300
	21st to end	Day	15% 7% 37% 25% 15% 1	330
February	1st to 10th	Day	15% 11% 40% 27% 6% 1	300
	11th to 20th	Day	13% 15% 38% 25% 7% 2	300
	21st to end	Day	16% 10% 39% 30% 5%	249
March	1st to 10th	Day	16% 11% 36% 27% 7% 2	300
	11th to 20th	Day	18% 8% 33% 27% 12% 2	300
	21st to end	Day	22% 12% 27% 25% 11% 3	355
April	1st to 10th	Day	21% 13% 28% 30% 7% 1	500
	11th to 20th	Day	26% 10% 26% 26% 12% 1	540
	21st to end	Day	24% 7% 31% 24% 12% 3	700
May	1st to 10th	Day	18% 7% 29% 31% 13% 3	703
	11th to 20th	Day	18% 8% 25% 31% 14% 4	725
	21st to end	Day	18% 5% 25% 27% 21% 4	789
June	1st to 10th	Day	10% 5% 27% 29% 25% 4	730
	11th to 20th	Day	16% 5% 30% 32% 15% 1	730
	21st to end	Day	14% 5% 26% 31% 16% 7%	730
July	1st to 10th	Day	9% 2 18% 34% 27% 10%	721
	11th to 20th	Day	9% 4 23% 33% 22% 9%	718
	21st to end	Day	8% 3 25% 34% 22% 8%	794
August	1st to 10th	Day	12% 4 20% 32% 25% 6%	726
	11th to 20th	Day	13% 4 19% 31% 26% 6%	715
	21st to end	Day	13% 6% 21% 31% 23% 5%	708
September	1st to 10th	Day	11% 5% 30% 29% 20% 5%	530
	11th to 20th	Day	11% 5% 25% 34% 21% 3	496
	21st to end	Day	18% 9% 26% 26% 14% 7%	330
October	1st to 10th	Day	19% 6% 32% 25% 12% 6%	330
	11th to 20th	Day	18% 16% 34% 25% 6%	324
	21st to end	Day	25% 10% 38% 19% 7%	330
November	1st to 10th	Day	21% 10% 30% 30% 9% 1	300
	11th to 20th	Day	19% 15% 38% 23% 5%	300
	21st to end	Day	20% 13% 39% 24% 4	300
December	1st to 10th	Day	14% 12% 39% 25% 10%	300
	11th to 20th	Day	18% 15% 38% 22% 6%	300
	21st to end	Day	27% 11% 33% 25% 3 1	330
January	1st to 10th	Night	12% 9% 36% 33% 9% 1	1000
	11th to 20th	Night	15% 6% 38% 31% 8% 1	1000
	21st to end	Night	15% 8% 38% 30% 8% 1	1100
February	1st to 10th	Night	11% 8% 40% 31% 9% 1	1000
	11th to 20th	Night	11% 10% 41% 28% 9% 1	979
	21st to end	Night	15% 9% 35% 30% 10% 1	786
March	1st to 10th	Night	15% 9% 36% 29% 9% 1	675
	11th to 20th	Night	16% 10% 40% 27% 8%	600
	21st to end	Night	21% 7% 30% 29% 9% 3	660
April	1st to 10th	Night	21% 8% 35% 29% 7% 1	600
	11th to 20th	Night	25% 8% 28% 31% 7% 1	593
	21st to end	Night	23% 6% 27% 33% 10% 1	600
May	1st to 10th	Night	16% 9% 33% 31% 11% 1	600
	11th to 20th	Night	17% 6% 30% 35% 10% 2	591
	21st to end	Night	16% 6% 24% 37% 16% 1	650
June	1st to 10th	Night	13% 5% 24% 41% 17% 1	600
	11th to 20th	Night	13% 3 31% 42% 12%	600
	21st to end	Night	13% 5% 21% 39% 21% 1	600
July	1st to 10th	Night	6% 2 17% 43% 28% 4	585
	11th to 20th	Night	10% 2 22% 40% 23% 3	582
	21st to end	Night	6% 2 20% 44% 24% 3	657
August	1st to 10th	Night	9% 2 17% 48% 21% 3	597
	11th to 20th	Night	14% 4 20% 35% 25% 4	594
	21st to end	Night	11% 5% 21% 37% 24% 2	660
September	1st to 10th	Night	10% 5% 22% 39% 21% 3	600
	11th to 20th	Night	11% 4 27% 37% 20% 1	600
	21st to end	Night	15% 7% 30% 30% 15% 2	673
October	1st to 10th	Night	18% 9% 30% 30% 13% 1	800
	11th to 20th	Night	21% 11% 34% 28% 6% 1	793
	21st to end	Night	24% 12% 30% 28% 6%	836
November	1st to 10th	Night	20% 10% 29% 31% 9%	942
	11th to 20th	Night	20% 12% 33% 26% 8% 1	1000
	21st to end	Night	16% 13% 38% 25% 8%	1000
December	1st to 10th	Night	15% 11% 35% 30% 8% 1	1000
	11th to 20th	Night	20% 9% 36% 27% 8% 1	1000
	21st to end	Night	25% 8% 33% 25% 7% 2	1100

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